A resistivity logging tool featuring a focused electrode is set forth in U.S. Pat. No. 3,772,589. In the operation of this device, it is desirable that current flow from the electrode array of the tool radially outwardly, penetrating deeply into the adjacent formation. The array flows current to a remote fixed current return electrode. The current flow away from the tool (suspended in the wellbore while making measurements) can be resolved into vector components. The desirable direction of current flow is radially outwardly deep into the formation. An undesirable direction of current flow is along the tool in the borehole or invaded formation, a direction perpendicular to the desired direction of current flow.
This vertical current flow is a source of error. It creates an error in obtaining the true resistivity of the undisturbed formation by measuring electrode voltage and current. Determination of the resistance of the formation through Ohm's Law obtains erroneous data if there is a vertical component in addition to the radial component.
The present invention sets forth improvements in the focused electrode resistivity logging tool to enable the measurement of resistivity to be based primarily on current flow radially outwardly away from the tool deep into the formation undergoing tests. The improved resistivity logging system set forth in the present disclosure in two embodiments incorporates additional electrodes on the resistivity logging tool to thereby create a controlled vertical current along the wellbore. This vertical current is systematically varied to reverse the current flow. By measuring the voltage and current flow between the electrodes which create the vertical current, zero crossing or nulling of the vertical current can be determined. At the instant when this occurs, all current flowing from the focused electrode system is radially into the formation, and formation resistivity can then be accurately determined by Ohm's Law.
There is a distinct advantage in utilizing this apparatus. Devices of the prior art are primarily feedback type systems. They are feedback systems including the earth's formation as an active component in the feedback loop. This imposes unusual performance requirements on the feedback system to compensate for the wide dynamic range of values of the components interposed in the feedback loop. Such feedback loops tend to be unstable; this apparatus avoids the difficulties of destabilized feedback loops.
With the foregoing in mind, the present apparatus is briefly summarized as alternate embodiments of a system for resistivity logging featuring a focused electrode system incorporating additional balanced electrode pairs creating a vertical current component flowing in the adjacent wellbore formation. This current flow is evaluated by measuring the current and voltage of the electrode pairs. A variable power supply, such as an oscillator, is connected to the additional electrode pairs. Periodically, the current passes through zero as it reverses directions which reversal is observed by a null detecting system.